Powder coating compositions have become increasingly desirable in recent years for various reasons including those relating to ecology, health and safety. More particularly, powder coating compositions are preferable to liquid paints containing volatile solvents which must be volatilized after application, thus resulting in solvents escaping into the atmosphere and creating health and safety hazards as well as undesirable pollution problems. Powder coating compositions, on the other hand, are curable by heat in a manner such that little, if any, volatile material is given off to the environment.
While powder paint compositions have been proposed, coatings formed from such compositions share various problems, most of which at least in part result from the processes by which they are prepared. Prior art manufacturing methods include ball milling, Z-blade mixing and extrusion, all of which suffer from certain deficiencies.
Ball milling is the simplest of these three methods. In the manufacture of epoxy based powder coatings, for example, all ingredients such as granulated epoxy resin, hardener or curing agent, pigments and other additives are loaded into a ceramic lined ball mill. Ceramic grinding media of varied sizes and shapes are generally used to grind the materials for ten to fifteen hours to produce a blend. While other types of ball milling can be used to produce a similar blend in a shorter time, the method is very time consuming and is not adaptable to a continuous process. Also, such a method does not achieve good dispersion of pigments and other additives such as crosslinking agents. Thus the powders formed in such a manner cure to form coatings of exceptionally low gloss and opacity. Still further, such a process does not allow satisfactory color and shade matching.
In Z-blade mixing, resin is first heated in a Z-blade mixer to at least its melting point and when molten all other ingredients except curing agent are added slowly. In general, dispersion requires about 6 hours after which the temperature of the Z-blade mixer is reduced and curing agent added. As soon as the curing agent has been adequately mixed, the melt is cooled, pulverized and classified. The compositions formed by this process generally produce coatings having inadequate pigment dispersion and gloss requirements for top coat uses such as for automobiles. As was the case with ball milling it is also difficult to obtain adequate color and shade matching with this process. Also, since the process is a batch process, the mill has to be thoroughly cleaned after each run because of the build-up of heat sensitive material.
In the extrusion method, the resin being used is blended with all the other ingredients and fed into a heated extruder where high shearing forces are applied to effect mixing of the viscous molten components. Although the powders produced by this method are generally of higher quality than those formed from powders made by processes including ball milling or Z-blade mixing steps, the process still suffers from several serious disadvantages. First, the high shear forces required to effect mixing may cause a loss of color match in the final powder. Second, in such a process the thoroughness of mixing as well as the speed with which an adequately mixed extrudate may be produced will depend on the amount of shear force applied to the material which in turn is a function of the speed at which the extruder screw turns. Thus, in order to achieve rapid, thorough mixing of the viscous resin and molten crosslinking agent the extrusion process has high power requirements and is, therefore, expensive. Third, the process is also expensive because of the relatively high cost of the extrusion equipment. Finally, because of the inherent output limitations on any given extruder, the process is not readily adaptable to varied processing speeds. Thus, the input into the extruder in such a process must be metered to the capacity of the extruder.
The processes disclosed in the applications Ser. No. 389,845 entitled "Method of Preparing Powder Paint Compositions-I" and Ser. No. 389,844 entitled "Method of Preparing Powder Paint Compositions-II", both filed Aug. 20, 1973, overcome many of the problems associated with these prior art processes by accomplishing more complete and uniform dispersion of paint additives. Both processes comprise introducing liquid compositions containing at least a cross-linkable copolymer and uniformly dispersed pigment into an evaporating zone adapted to evaporate the inert solvents contained therein; passing the liquid into a separating zone; causing the solvent vapor to be removed; removing the non-volatile components of the paint in a molten state by gravity; cooling the nonvolatile components; and pulverizing to form a powder. The former application covers processes wherein, if desired, either a self-crosslinking copolymer or a crosslinkable copolymer and a crosslinking agent therefore are included in the liquid paint composition and processed directly through evaporating and separating zones prior to cooling and pulverizing. Such a process has obvious advantages in that all additives including crosslinking agents if employed are uniformly dispersed in the molten resin. However, such a process has the disadvantage that while the molten material is being withdrawn from the separating zone, premature crosslinking may occur. The process of the latter application avoids this problem by processing liquid compositions which contain crosslinkable copolymers but not crosslinking agents and then mixing the powder and crosslinking agent at a temperature greater than the melting point of both components, cooling the resultant molten homogeneous mass to form a solid and then pulverizing to form the final powder. While this process eliminates premature crosslinking the equipment involved is expensive and the process is inefficient because of the number of steps involved.
Accordingly, it is an object of this invention to provide a process of forming powders wherein complete and uniform dispersion of all paint additives including crosslinking agents is effected efficiently in a continuous operation which may be adapted to various processing speeds and at a minimum of expense without resulting in composition degradation or premature crosslinking.